305/40R20 vs 325/35R20
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R20 | 325/35R20 | Change |
|---|---|---|---|
| Overall height | 29.6" | 29.0" | -2.2 % |
| Sidewall | 4.80" | 4.48" | -6.8 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 93.0" | 91.0" | -2.2 % |
| Revs per mile | 681 | 696 | +2.2 % |
| Axle height | 14.80" | 14.48" | -8.3 mm |
| Approved rim width | 8.5–11" | 9–12" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/40R20. Fit 325/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,781 on the road, so it over-counts by 219. At a true 70 mph the first turns 795 times a minute, the second 813.